Microwave and millimeter-wave (MMW) communication systems are commonly constructed with various components and subcomponents such as receiver, transmitter, and transceiver modules, as well as other passive and active components, which are fabricated using MIC (Microwave Integrated Circuit) and/or MMIC (Monolithic Microwave Integrated Circuit) technologies. The system components and/or subcomponents can be interconnected using various types of transmission media such as transmission lines (e.g., microstrip, slotline, CPW (coplanar waveguide), CPS (coplanar stripline), ACPS (asymmetric coplanar stripline), etc.) or coaxial cables and waveguides.
Microstrip transmission lines are commonly used in radio frequency (RF) CMOS/SiGe chips, where wiring is not dense. On the other hand, coplanar waveguides are commonly used where wiring density is relatively high, such as in CMOS chips, for example, where it is difficult to create an explicit return path below the signal line. A third structure referred to as a microstrip transmission line having side shielding (i.e., having characteristics of both microstrip and coplanar structures) has also been used in existing transmission line structures.
The characteristic impedance (Zo) of a transmission line can generally be thought of as the square root of the ratio of inductance (L) to capacitance (C), that is, Zo=SQRT (L/C). In some applications, it is desirable to have a relatively constant characteristic impedance. For example, a constant characteristic impedance reduces the severity of impedance-mismatch between two adjacent transmission structures. Impedance-mismatch can disadvantageously result in undesired characteristics such as reflections, ringing, etc. For example, changes in impedance along a transmission path can result in energy being reflected or dispersed.
However, in conventional microstrip transmission lines, the characteristic impedance varies with signal frequency. This is because the inductance varies with frequency, while the capacitance remains relatively constant across a wide range of frequencies. As a result, conventional microstrip transmission lines normally do not exhibit a relatively constant characteristic impedance over a wide range of signal frequencies. Therefore, it is difficult to optimize a transmission line to operate at a constant Zo over a wide range of frequencies.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.